Molten aluminum and molten zinc have been used for years to coat the surface of steel. One of the coating process steps is to immerse the steel sheet in the molten aluminum or molten zinc. The surface quality of coating is very important to produce high quality coated products. However, introduction of aluminized steel for the US market in 2007 was quite a challenge for the aluminizing lines. Early trials resulted in >50% rejects due to coating defects.
One of the major sources of defects was dross floating on the aluminum bath within the snout and sticking to the strip. To achieve high quality surface finish, floating dross and oxides in the molten metal bath, especially in the confined regions inside the snout, need to be diverted from the surface being coated. Carbon steel pneumatic dross pump, also referred to as bubble pump, has been used to remove the dross from the coating zone. Implementing push and pull snout pumps to ensure a dross-free melt surface inside the snout made high quality coating possible. The bubble pump (a.k.a. dross pump) uses the artificial lift technique of raising a fluid such as water or oil (or in this case molten metal) by introducing bubbles of compressed gases, air, water vapor or other vaporous bubbles into the outlet tube. This has the effect of reducing the hydrostatic pressure in the outlet tube vs. the hydrostatic pressure at the inlet side of the tube. The bubble pump is used in the molten metal bath of the metal coating lines to remove floating dross from surface of the aluminizing bath inside the snout in order to prevent dross-related defects on the coated strip. Thus, the bubble pump is a critical hardware component in the production of high quality automotive aluminized sheet.
One of the major factors impacting production costs is aluminizing pot hardware failures. Prominent among hardware failures is the failure of the bubble pump (pull pump). The average service life of bubble pumps made of carbon steel is 8-12 hours, resulting in the use of 35-40 pumps every month (for a 2 week production). The change of carbon steel bubble pumps during production leads to production disruption and contamination of molten metal bath. In addition, the “quality” of the coated steel sheet must be downgraded (resulting in a less valuable product) during carbon steel pump changes. Further, pump changes require line stops and restarts, leading to excessive consumption of startup coils. Average losses attributable to bubble pumps are about close to a million U.S. dollars per year. An increase in life of the bubble pump will significantly reduce the quantity of downgraded sheet, and will reduce downtime and costs.
Thus, there is a need in the art for bubble pumps for use in molten aluminum baths that can last significantly longer than bare carbon steel tube pumps.